1. Field of the Invention
The instant invention relates to a peripheral sealing device for an axial compressor, specifically such a device wherein a seal ring defining a plurality of grooves on its inner surface is disposed about the rotating rotor blades.
2. Brief Description of the Prior Art
Achieving an effective seal between the periphery of the rotating blades and the stationary casing of an axial compressor is a fundamental necessity for achieving and maintaining high operational efficiencies. While such peripheral leaks in turbines result only in a loss in efficiency affecting the specific fuel consumption, such leaks in compressors of the axial type generate secondary phenomena that strongly affect the stability of the flowing gases and the stability of the compressor itself.
It is well known in the art to attach abrading material to the interior of the stationary casing such that it is worn away by the tips of the rotor blades so as to minimize the clearance between the blade tips and the stationary casing. However, during the run-in period for such compressor blades, the blades can also be worn or deformed at their ends during their centrifugal elongation resulting in a detachment of the flow of the gases passing over the casing and introduces a random friction energy into the blading which may induce periodic vibration corresponding to the critical period of the rotating mass.
It is further known to form the tips of the rotor blades in a step-like formation and to machine a corresponding formation into the wall of the stationary casing. Such a system is shown in French Pat. No. 1,218,301. Generally, the interior wall of the casing converges in step-like fashion in the direction of the flow such that the leak flow current indicent on the vertical wall of the step is deflected in a tangential direction. While this system is somewhat effective, it requires delicate machining of both the turbine blades and the stationary casing, thereby resulting in a high manufacturing cost.
The system shown in French Pat. No. 2,432,105 eliminates the necessity of forming the step-like formation on the rotor blade tips and merely forms a plurality of discontinuities, in the shape of circumferential grooves or axially extending cavities in the casing wall adjacent to the blade tips. Due to the thermal and centrifugal expansion of the rotor blade and rotor blade wheel, the tips of the blades enter into the cavities during operation to seal the gas flow. However, experience has shown tht this system does not markedly improve the efficiency of the device, due to the detachment of the gas flow produced on the descending step of the peripheral passage. This system also introduces additional complex mechanical problems due to the axial displacement of the rotor blade and wheels which may cause the blade tips to contact the stationary casing.
The sealing system shown in French Pat. No. 2,440,467 utilizes a plurality of generally radially extending circumferential grooves in the casing adjacent the rotor blade tips to thicken the boundary layer so as to counteract the leakage flow. However, it has been found that increasing the thickness of the boundary layer entails greater susceptibility to the detachment of the gas flow, which is undesirable.